Congestive heart failure, or simply heart failure, is a condition in which a subject's heart can't pump the needed amount of blood to the subject's other organs causing fluid to build-up behind the heart. Congestive heart failure can be conceptualized as an enlarged weakened heart muscle, which results in poor cardiac output. As blood flow out of the heart is reduced, blood returning to the heart through the veins backs up, causing congestion in bodily tissues. This congestion may cause swelling in the legs, ankles, or other parts of the body and may also result in fluid collecting in, or flowing to, the subject's thorax. Thus, congestion may be associated with heart failure.